Rates of nutrient removal among the highest ever recorded

A
study led by researcher Lisa Kellogg of William
& Mary's Virginia Institute of Marine Science shows that a restored oyster
reef can remove up to 10 times more nitrogen from Chesapeake Bay waters than an
unrestored area nearby, providing additional evidence that reef-restoration can
contribute to efforts to improve water quality in the nation’s largest estuary.

To
date, the justification for restoring oysters to Chesapeake Bay has focused on
their capacity to clear the water, provide habitat for their own young and for
other species, and to sustain both watermen and seafood lovers.

The
new study, says Kellogg, aimed to quantify another potential benefit of
restored oyster reefs—their ability to remove nutrients from the water. Input
of nitrogen and phosphorous from fertilizers, wastewater treatment plants, and
other sources is one of the main reasons for impaired water quality in the Bay,
with reduction and removal of these excess nutrients a key goal of Bay
restoration efforts.

In
Maryland’s Choptank River, the team compared a restored oyster reef that had
131 large oysters per square meter to an adjacent unrestored site that was
suitable for restoration. They carefully measured flows of nitrogen and
phosphorous compounds at each site, along with salinity, temperature, oxygen
levels, sediment characteristics, and the abundance of oysters and other marine
life such as mussels, clams, barnacles, and worms.

“Our
study showed that a successfully restored oyster reef can remove significant
levels of nutrients from the water column,” says Kellogg. “We found that annual
denitrification rates at the restored site were enhanced by an order of
magnitude and that rates in August were among the highest ever recorded for an
aquatic system. It’s important to recognize, however, that the density of
oysters on the reef we studied far exceeds current success criteria for
oyster-reef restoration.”

Denitrification—the
conversion of reactive nitrogen compounds back into the inert nitrogen gas that
makes up most of our atmosphere—is a holy grail of restoration science. In its
pure, gaseous form, nitrogen contributes little or nothing to the
over-fertilization that plagues coastal waters worldwide.

Uptake
into the tissues and shells of living organisms provides another way to remove
nitrogen compounds from the water. The team’s research shows that the restored
reef provided habitat for almost 25,000 bottom-dwelling organisms per square
meter compared to just over 2,000 organisms per square meter at the control
site. Nitrogen in shells of live oysters and mussels accounted for 47% of all
nitrogen incorporated into living organisms at the site.

Kellogg
says incorporation into shell is particularly important because “the shells of
oysters and mussels can persist long after the organisms die, removing
nutrients from the water for years, decades, or even centuries if the shells
become buried.” Nutrients in soft tissues, on the other hand, can be quickly
recycled back into the water, once again contributing to over-fertilization.

State
and federal officials have used spat-on-shell and other methods to restore
oyster reefs at more than 500 sites in Chesapeake Bay and Maryland and
Virginia’s seaside bays during the last few decades. Although the high
densities of oysters used in the study fall well within the range of historic
oyster densities, these densities are rarely achieved today, due in part to the
limited resources available for oyster-reef restoration. Kellogg says “The
expense of oyster-reef restoration, especially when hatchery-produced juvenile
oysters are used, raises the question of whether its benefits are worth the
investment. Our results add to the growing body of evidence suggesting that the
benefits do outweigh the costs.”

Putting
their findings into a management context, the team used their results from the
Choptank reef to estimate the scale of oyster restoration needed to meet the
EPA’s new nitrogen restrictions for this Chesapeake Bay tributary. “We estimate
that more than 475,000 pounds of nitrogen need to be removed from the Choptank
to meet current restrictions,” says Kellogg. “According to our calculations,
this could be done by restoring high-density oyster reefs to 23% of the river’s
available bottom.”

The
researchers temper their conclusions by noting that additional work is needed
to test whether their results from the Choptank apply equally to other parts of
the Bay with different environmental conditions. They also stress that oyster
reef restoration is not a substitute for reduction in land-based inputs, but
rather a potential safety net to reduce additional downstream impacts. Most of
the excess nitrogen in the Choptank River enters in low-salinity areas that do
not support oyster growth, and will thus still have negative impacts upstream
of restored oyster reefs.